Olefin homopolymers and copolymers have become of commercial importance for the manufacturers of numerous shaped articles and other utilities. Unfortunately the olefin homopolymers and copolymers are often translucent due to the crystal structure of the polymer. Improvement in the clarity of the homopolymer and copolymer can be obtained by the use of nucleating agents which enhance nucleation rather than crystal growth during solidification of molten polymer. The use of nucleating agents causes the formation of smaller crystals and accordingly polyolefins having improved clarity and physical properties. In addition the nucleated polyolefins have higher crystallization temperatures. Consequently nucleating materials are of significant importance in improving the value of the polyolefins.
The requirements for an effective nucleating agent are difficult to meet particularly since it must be effective at low concentrations. In order to function as an effective nucleating agent, the compound must be partially insoluble in the polyolefin. On the other hand in order to achieve clarity, the nucleating material must be transparent in the polyolefin at the concentration it is used as a nucleating agent.
Among the other characteristics of a nucleating agent are good thermal stability and the substantial absence of odor, particularly at the melt processing temperatures of the polyolefin. Low melting points and good dispersibility are important in improving mixing of the nucleating compound into the molten polyolefin.
An object of the present invention is a new nucleating agent for polyolefins.
Another object of the present invention is a polyolefin having improved clarity.
Still another object of the present invention is a shaped article from polyolefins which are substantially transparent.
Also an object of the present invention is a method for improving the clarity of polyolefins.
A particular object of the present invention is an improvement in the process for the preparation of polypropylene and polyethylene polymers and copolymers and mixtures thereof which are substantially transparent.
Other objects will become apparent from the ensuing description.
Many materials have been proposed for use as nucleating agents with some success. Among these disclosures are:
U.S. Pat. No. 3,207,735 discloses the use of benzoic acid type compounds to improve the properties of polypropylene.
U.S. Pat. No. 3,207,738 discloses the crystallization of polypropylene using aryl alkanoic acid type compounds.
U.S. Pat. No. 3,268,499 discloses the crystallization of polymers such as polypropylene in the presence of lithium salts of carboxylic acids.
U.S. Pat. No. 3,299,029 discloses the crystallization of polymers such as polypropylene in the presence of aluminum salts of carboxylic acids.
U.S. Pat. No. 3,517,086 discloses aromatic sulfonic acids as nucleating agents.
U.S. Pat. No. 4,704,421 discloses benzoic acid and adipic acid as nucleating agents for propylene polymers.
U.S. Pat. No. 4,801,637 discloses the use of an alcoholic solution of an aromatic carboxylic acid as a nucleating agent for certain crystalline propylene homopolymers or copolymers.
U.S. Pat. No. 4,829,114 discloses a combination of carboxylic acids which contain at least three carbon atoms and an amine such as an ethoxylated amine as nucleating agents for polyolefins.